Conventionally, optical components such as an optical lens and a prism arc used for optical apparatuses such as a camera, a microscope and a telescope; electronograph system recording apparatuses such as a printer and a copier; an optical recording medium such as DVD; and an optical device. For example, it is necessary to make the beam diameter of a laser beam for recording as small as possible so as to raise recording density in the field of an optical recording. Thus, in order to focus shorter wavelength light efficiently, a lens and a prism that retain high optical transmittance to a short wavelength band with a high refractive index, without anisotropy, is necessary.
For example, the beam spot diameter of a laser beam is determined by a wavelength λ of a light source and a numeric aperture NA of a lens, and is known as 0.8×λ/NA. An information capacity of 4.7 GB can be recorded in a conventional DVD recording apparatus having a 5-inch disk using a semiconductor laser with a 650 nm wavelength and a lens having NA=0.6. Recently, DVD (Blu-ray) recording apparatuses with enlarged NA and decreased wavelength have been developed. These apparatuses have realized a storage capacity of about 23 GB in a 5-inch disk using a semiconductor laser with a 405 nm wavelength and a lens having NA=0.85 as a light source.
Additionally, a near field recording method is known to raise recording density by Evanescent light at the reflecting surface using a minute lens with a high refractive index. This minute lens is a hemispheric lens called SIL (Solid Immersion Lens) and is placed between an optical recording medium and an objective lens. In such an optical system, the beam spot diameter of a laser beam transmitted through the objective lens equivalently becomes λ/(nk×NA) (n is the refractive index of a SIL) and can focus to 1/n compared to those without a SIL. (e.g., See Nonpatent Document 1). In the area where the gap between the recording surface of the optical recording medium and the bottom surface of the SIL is below ¼ of the light wavelength, a laser beam transmitted through the SIL is outputted in a same disposition as an interior of the SIL. Then, the beam spot diameter is focus to 1/n of a diffraction limit.
Above referenced SIL becomes NA=n2sin θ when incident angle is θ and has a great influence on the refractive index of an optical medium. For this reason, the optical medium with a high refractive index is crucial and an isotopic optical medium without birefringence is crucial at focal power. In addition, it is important to be an optical medium whose optical transmittance does not deteriorate to the short wavelength band. Beside a SIL, optical components such as a camera, a microscope and a stepper have implemental constraint and, a small lens with a high refractive index and high focal power is necessary. If the lens is of the same size, large NA, high focal power and bright lens is preferable. In the same way, a prism may be realized small and sufficient spectroscopic characterization by using high refractive index materials.
Concerning the above mentioned viewpoint, high refractive index glasses and crystal materials have been examined. For the glasses, high refractive index glasses containing much La and Pb, and TeO2-based glasses are known. However, an optical medium that can realize a refractive index of 2.2 in a visible wavelength band has not been developed. There is a problem in that optical transmittance at around 400 nm deteriorates by raising refractive index.
Meanwhile, concerning the crystal materials, a lot of lenses with a high refractive index have been developed by using oxide crystals (e.g., See Patent Document 1). However, optically isotropic materials without birefringence are limited to cubic crystal structure. Although, many crystals are disclosed in the Patent Document 1, isotopic materials are limited to SrNbO3, SrTaO3, Bi20SiO12, Bi20GeO12, Bi4Ge3O12 and GaP. Concerning these crystals, the refractive index in the visible wavelength band is from 2.06 to 2.22.
In addition, high-intensity liquid crystal projectors require a prism for a polarization optical system, which has high optical transmittance of the visible wavelength band without anisotropy of refractive index, and has a high refractive index. Although borosilicate glasses were used conventionally, there was a disadvantage that photo-elastic effect is large. Thus, it is examined to use high refractive index glasses such as lead-containing glasses. However, there has been a problem that these glasses have low transmittance in the short wavelength band, and cannot be used in the wavelength band used for the liquid crystal projector.
Furthermore, an air pollution measuring device has no absorption up to a long wavelength band of around 5 μm, and requires optical materials with a high refractive index. Conventionally, optical transmittance in the long wavelength band of the optical medium is as follows. An optical transmission range of silica based glasses is up to around 2 μm and silica based glasses have a low refractive index. Fluoride glasses such as ZBLAN (ZrF4—BaF2—LaF3—AlF3—NaF) have high optical transmittance but the refractive index is low around 1.5. Calcogenite glasses such as Ge—Sb—Se have high optical transmittance but have problem in toxicity. Therefore, optical medium with high optical transmittance in the long wavelength band and a high refractive index is required.
Patent Document 1: Japanese Patent Application Laying-Open No. 2000-19301
Nonpatent Document 1: Sharp Technical Report “Trend of high-capacity optical disk” Vol. 72, December, 1998, pp. 9-12